DOCBOOK: Modularity and Emacs psgml mode

I am adopting the common layout which uses system entities to split
the work in more manageable files, like:

<!doctype book public "-//OASIS//DTD DocBook V3.1//EN" [
<!entity chapter1 system "chap1.sgml">
<!entity chapter2 system "chap2.sgml">
]>

<book>
&chapter1
&chapter2
</book>

This means that I cannot begin chap1.sgml and chap2.sgml with:

<!doctype chapter public "-//OASIS//DTD DocBook V3.1//EN" []>

Because, of course, the file will be included and it cannot present
such a line in the middle of the document.

However, this also means that I cannot edit chap1.sgml and chap2.sgml
taking advantage of Emacs psgml mode, because it won't find any
identifier/catalog.

Is there any way to overcome this?

Thanks for any input.

Massimiliano